Imaging spectrograph for multiorder spectroscopy

Abstract

The Ebert-type mounting is modified for use as a multiorder spectrograph, by replacing the spherical primary mirror of the Ebert with a paraboloidal mirror to eliminate the astigmatism and spherical aberration of the Ebert mounting, and by replacing the Ebert's rotating plane grating, normally blazed for use in the first order, with a fixed low-blaze-angle grating blazed at a longer wavelength such that the radiation at the shorter wavelengths, for which the grating will be used, will be most efficiently dispersed into a multiplicity of higher spectral orders. In a preferred embodiment of this invention, these spectral orders are separated using a twice-through cross-dispersing prism mounted near the grating surface, with the grating and prism mounted and aligned together in a crossed-dispersion assembly that is interchangeable with other crossed-dispersion assemblies containing other grating and prism combinations.

What I claim is:  
     
       1. A cross-dispersed low-blaze-angle multiorder plane grating spectrograph using a modified Ebert-type mounting comprising 
       a single concave paraboloidal mirror having a focus, the mirror replacing the single spherical primary mirror of the Ebert-type mounting;  
       a plane diffraction grating, the grating having equally-spaced grooves, the grating located near the focus of the paraboloidal mirror, the grating having a blaze angle, the blaze angle less than 20°;  
       a cross-dispersing element, the cross-dispersing element located between the paraboloidal mirror and the grating;  
       a detector array, the array containing pixels in a multiplicity of rows and a multiplicity of columns, wherein a cross-dispersed spectral image is located at the array, the spectral image containing a multiplicity of spectral orders, the orders separated from each other by the cross-dispersing element.  
     
     
       2. The spectrograph as defined in  claim 1  wherein the cross-dispersing element is a prism, the prism having an apex, the apex perpendicular to the grating grooves. 
     
     
       3. The spectrograph as defined in  claim 2  wherein the prism is mounted almost in contact with the grating, whereby radiation incident on and reflected from the grating passes twice through the prism. 
     
     
       4. The spectrograph as defined in  claim 2  wherein the prism and grating are mounted together in a crossed-dispersion assembly, the crossed-dispersion assembly interchangeable with other crossed-dispersion assemblies containing a prism and a grating. 
     
     
       5. The spectrograph as defined in  claim 2  wherein a fluid of refractive index n fills the space between the prism and the grating, whereby the blaze angle of the grating is effectively increased by a factor of n.